Lithium-ion secondary batteries with high energy densities have been used as power sources for portable electronics such as a mobile phone and a notebook computer.
An electrode member of a lithium-ion secondary battery includes a positive electrode plate, a separator, and an anode plate. Regarding a positive electrode material, an aluminum alloy foil has been used as a support, having excellent electrical conductivity and less heat generation without affecting electrical efficiency of a secondary battery. An active material having a lithium-containing metal oxide such as LiCoO2 as a chief component is applied on a surface of the aluminum alloy foil. Its production process includes: applying an active material with a thickness of about 100 μm on both sides of an aluminum alloy foil with a thickness of about 20 μm; and drying the active material to remove a solvent therefrom. Further, in order to increase the density of the active material, compression forming is performed with a pressing machine (hereinafter, this step is referred to as press working). The positive electrode plate as so manufactured, a separator, and an anode plate are stacked, and then the resulting stack is wound. After a shaping process is performed so as to encase the stack, it is encased.
An aluminum alloy foil used for a positive electrode material of a lithium-ion secondary battery has several problems that cuts occur during application of an active material and that ruptures occur at a bending portion during winding. Thus, a higher strength is required. At a drying step after the application of the active material, in particular, heat treatment is carried out at about 100 to 180° C. Accordingly, a lower strength after the drying step is likely to generate middle waviness during press working. This induces wrinkles during winding, which reduces adhesion between the active material and the aluminum alloy foil. Besides, a rupture is likely to occur during a slitting process. When the adhesion between the active material and a surface of the aluminum alloy foil decreases, their detachment is facilitated during repeated operation of discharge and charge. Unfortunately, this causes its battery capacity to decrease.
Recently, a high electrical conductivity has also been required for an aluminum alloy foil used for a positive electrode material of a lithium-ion secondary battery. What is meant by the electrical conductivity refers to physical property indicating how easily electricity is conducted in a substance. The higher the electrical conductivity is, the more easily the electricity is conducted. Lithium-ion secondary batteries used for automobiles and/or electric tools necessitate a higher output characteristic than lithium-ion secondary batteries used for consumer-use mobile phones and/or notebook computers. When a large current flows, a lower electrical conductivity causes internal resistance of a battery to increase. Consequently, this reduces its output voltage.
Meanwhile, 3003 alloy is generally used as a strong aluminum alloy foil for a lithium-ion secondary battery. The 3003 alloy primarily contains elements such as Si, Fe, Mn, and Cu, and is thus characteristic of high strength. Particularly because Mn is added, a solid solution amount of Mn is large. In addition to solid-solution elements that can prevent dislocation movement, fine precipitates are also present, so that its strength decrease is small. Mn, however, decreases electrical conductivity when forming solid solution. Therefore, the 3003 alloy is much lower in electrical conductivity than aluminum alloy whose Al purity is 99% or more. That is, the 3003 alloy has difficulty achieving both high strength and high electrical conductivity which are required for an aluminum alloy foil for a lithium-ion secondary battery.
Patent Literature 1 discloses an aluminum alloy foil with a tensile strength of 98 MPa or more, which foil is used for a battery current collector. Patent Literature 2 discloses an aluminum alloy foil with a tensile strength of 200 MPa or more, which foil is used for an electrode current collector of a lithium-ion secondary battery. Both Patent Literatures 1 and 2, however, disclose nothing about electrical conductivity.
Patent Literature 3 sets forth a method for preventing detachment from an active material without inducing plastic deformation during press working by increasing its strength. However, the alloy used contains Mn, Cu, and Mg as principal elements. Therefore, it is impossible to achieve a high electrical conductivity.